This invention relates generally to under-load switching devices, and more particularly to under-load switching devices useful for regulation and balance of AC voltages.
There are many electrical systems that are sensitive to voltage changes. AC powered systems may be very sensitive to variations in the AC voltage level supplied to the system, and multi-phase systems may be very sensitive to unbalances of or differences between the phase voltages.
Voltage variations and phase unbalance are problems in oil well pumping systems employing electrically driven submergible pumps. The motors used for driving submergible pumps are generally three-phase AC motors having a somewhat unusual design. They have a very small diameter and a relatively long length. Their large length-to-diameter ratio combined with a hostile downhole environment imposes very severe duty on such motors. If the severe duty is coupled with poor regulation of the AC voltage level and poor voltage balance between phases, the life of the motor may be severely reduced. It is desirable, therefore, to regulate the AC power in order to maintain relatively constant voltage levels and balance between phase voltages as the load and other system conditions change. Furthermore, it is desirable to perform such regulation and balance under load conditions.
There are devices that are capable of regulating and balancing AC voltages and that do not require voltage switching. Phase controlled rectifiers may be employed for converting the AC voltage to a DC voltage, which is regulated and then reconstituted as an AC waveform using an inverter circuit. Controlled variable impedances, such as saturable core reactors, may be inserted in series with the AC lines and level regulation and balance between phases achieved by varying the voltage drop across each impedance to compensate for variations in voltage levels or phase voltage unbalances. Such devices are complex and costly, and have other disadvantages.
Tapped power transformers may be employed for voltage regulation and balance purposes; however, tap changers, i.e., switching devices, are necessary for switching between voltage taps. Known tap changers generally comprise a plurality of switches connected to the voltage taps and to the AC line feeding a load through a plurality of current-interrupting switches. In order to operate under load, the tap changers are generally of the step-switching type, wherein a tap changing operation involves the sequential selection of adjacent taps, i.e., adjacent voltage levels, and the momentary connection of the adjacent taps together. Various protective devices such as auto-transformers or current-limiting impedances are necessary to reduce the circulating currents flowing between the connected taps. Such tap changers generally do not permit the independent selection of non-adjacent taps in a single tap-changing operation, but require a series of steps in which adjacent taps are sequentially selected until the desired tap is reached.
Tap changers of the step-switching type just described require multiple switches and require special transformers having an extra winding for the protective auto-transformer (otherwise an external auto-transformer or current-limiting impedance is required). However, such transformers are not generally available in the sizes required for submergible pump installations. In addition to being relatively complex and expensive, step-switching tap changers, requiring multiple switching operations to switch between non-adjacent taps, are not capable of quickly responding to variations in voltage level and have other disadvantages.